Is there a correlation between a lean condition (chugging momentarily before taking off like bat our of hell) and an inconsistent idle speed? Still learning about carbs

JamesC

 

Both could be related to a vacuum leak.
However I'll ask:
By inconsistent idle speed do you mean the RPMs rise and fall or is it more case of idling at one RPM this day and then another RPM later that day?
And your "chugging momentarily before taking off like bat our of hell" symptom, is this from a standstill and matting it?
How have you determined that it's a lean condition? Do you use a wide band O2 gauge?

 

I know a loping idle can be a stretched timing chain..

 

There can be, yes.

Most often related to the transition system, in some form or fashion. On improperly tuned Holleys, often caused by the pri throttles set too high, which drains the entire system during idle, instead of it being full and ready to deliver fuel during tip-in.

The question about mixture is a good one. Of course back in the day before WBO2s were invented, there were other ways of answering basically the same question; but, the question is, is it rich, lean, or near right?

 

No vac leak that I'm aware of.
Idling at one RPM at one point during the day and then another RPM later.
Chugging at speed when I go to WOT.
No determination.

The pri slot is at .20. I've done your Holley tune. Rich (well, it passes the eye and nose test, if that means anything), so....

BTW, this ordeal started after I let the car sit too long over the winter. Maybe a tear down and thorough clean would be a good start.

JamesC

 

Always a good idea... some of your garage wildlife might have found a nice secluded and well-protected place to crawl up into and curl up and die.

 

I've experienced the "idle here now and idle there later" before. In one case it was a question of the throttle not returning completely. Often a little stab on the gas pedal would remedy it. Aggravating though. I'm running dual throttle springs full time now. The other time (and this is more or less my current situation) has no explanation that I can find. The variation is very slight. Maybe 150 rpm. Vacuum is solid. So is the mixture. I'm putting it down to a tired carburetor. Perhaps the throttle shafts are a little worn.
That of course is probably of little use to you and it still doesn't address the chugging you experience at cruise. Which, by the way, is very symptomatic of a lean condition. This is taking into consideration that the timing curve is what it should be.

And a rebuild eliminates the internal malfunction variable. Wildlife included.

 

Well, I disassembled the carb (4160) and shot it out with carb cleaner and compressed air. I'm using Sofa's Holley tune from a while back (reset the primary trans slot at .20 this time). The car runs strong with good response but the idle is stuck at a 1000. The secondary stop screw has no effect. My first thought was vac leak. I eyed the hoses and saw nothing. The carb base plate gasket is new. I'm open to suggestions.

JamesC

 

Have you thrown a vacuum gauge on it and tried the infamous propane leak detector? I found a leak at the back of my carb at the base gasket that way. Turns out the carb and carb pad on the manifold weren't a perfect fit. A new gasket was fine for months but failed over time.

 

Old gasket, new gasket made no difference. Vac and RPM are steady.

JamesC

 

possibly forget to hook up a vacuum line?

is it fully returning to the idle posistion? like, if you pull it back with your hand, does it idle down?

 

I've double-checked everything I can think of. Sometimes when I start the car, the idle will be where it should be at around 800-850, but as soon as I hit the throttle, idle rises to the 1000-900 mark. I've inspected the throttle linkage and thoroughly shot it with carb clean.

JamesC

 

Is it possibly hanging up on the choke fast idle cam?

All guesses here but someone might nail it.

 

Leak around the throttle shafts? I've had that issue before.

 

Messed with it today. No improvement. Thanks for the guesses—that's all I've got at the moment.

No leaking. I'm still curious about why there's no adjustment using the secondary idle stop screw.

Frustrating!

JamesC

 

hows the float levels look?

 

On the money.

JamesC

 

Had the exact same issue with both my holley and quickfuel, solved by closing the secondary throttle blades a little. They "squared" the transition slot, but still behaved like yours did. I closed em little by little untill it idled down.

 

This afternoon I found that I could raise the idle by turning out the secondary idle stop screw (I've installed an allen screw in the top of the base plate), but after a certain point in turning the screw in (900-1000 rpm), nothing happens. Blades stuck in the bore or something?

JamesC

 

Possibly, pull your carb off and check the position of the secondaries. Since you said it will occasionally idle down to 800 or so, its sounding like either the throttle blades are catching or opened too far, or the fast idle cam is catching or not fully disengaging.

 

Yes they could be stuck.

Turn the carb over and take off the throttle plate. Make sure the actual discs of the throttles fit squarely and straight in the bores. You can loosen the screws just enough to free them in the slot in the shaft (they're staked over so they won't back out very far) and fine-tune the alignment of the discs to the bore. Make sure they're good and tight when you finish.

Might want to check your PCV valve and make sure it's moving freely inside too; maybe, as cheeeeeep as they are, just replace it.

 

When I first installed my 4160 I did as the manual stated and ran a 12v (ign on only) wire to the choke, BUT I ran it through a toggle switch under the dash. The reason being because at the time I was chasing electrical demons and didn't want the choke heater sitting there heating the fuel while I had the ignition on. The reason I'm saying all of this is because if I leave the switch off, with no voltage to the choke heater it idles FAST. Turn on the choke switch and it returns to normal after a minute. Maybe your choke heater isn't getting voltage? Check with a test light maybe?

 

Well, the genius said with red face...I believe I may have set the primary transition slot with the fast idle cam in place. Is that possible? At any rate, this pm I threw caution to the wind and turned out the curb idle speed screw—and yep, the idle came down. So...I may start the process again, though the car runs pretty darned good. We'll see. Happy, but tired at the moment (old guy syndrome).

JamesC

 

Keep us posted.

 

Well, the car seems to like the current T-slot adjustment—whatever it may be. No gassy, eye-watering smell, good idle and vac. The throttle response is good, though that may require a bit of zoom zoom on my favorite tune road.

JamesC

 

Excellent (and easy on that "tune" road (we all have one don't we?)).

 

Well OF COURSE...

Gotta approach this whole deal scientifically. Gotta verify that the results are significant, repeatable, and occur under a wide range of conditions. So you know what that means... get up off yer keyboard and gather the data. Go do the research.

 

JamesC